Color has typically been imparted to optical products such as automotive and architectural window films by use of organic dyes. More particularly, the current commercial practice for producing dyed film from polyester involves swelling of the molecular structure of the substrate in baths of hot organic solvent such as ethylene glycol during the dyeing process, as swelled polyester (particularly PET) films are capable of absorbing organic dyes. These films and their manufacturing process suffer many drawbacks. Firstly, the substrates require exposure to organic solvents and elevated temperatures, which present both mechanical and chemical challenges such as environmental hazards and costs associated with storing the raw solvents and disposing of the resulting waste. Further, swelled substrates require special handling to avoid downstream stretching thereby decreasing the production yield. Next, the polyester elevated process temperatures and residual solvents in the substrate film after drying constrain downstream use and processing of substrates which in turn limits the potential end-use applications for such dyed films. On the process side, the existing methodology uses large volume dye baths which makes rapid color change within commercial manufacturing difficult. Finally, only a limited number of organic dyes are soluble and stable in the hot solvent swelling media and many of those are often subject to degradation by high energy radiation (sub 400 nm wavelength) to which the substrate is subjected when used in window film applications, thereby shortening the useful lifetime of the product.
To address these drawbacks, some film manufacturers have transitioned to using a pigmented layer on the surface of a base polymeric film for tinting a polymeric film. For example, U.S. Published Application number 2005/0019550A1 describes color-stable, pigmented optical bodies comprising a single or multiple layer core having at least one layer of an oriented thermoplastic polymer material wherein the oriented thermoplastic polymer material has dispersed within it a particulate pigment. As noted in this published application, these products can suffer myriad processing and performance drawbacks. For example, layers of this type are typically applied as thin films and can employ a relatively high pigment concentration to achieve a desired tint level, particularly in automotive window films with a relatively high desired level of darkening such as those with an electromagnetic energy transmittance in the visible region (or Tvis) of less than 50%. These high pigment concentrations are difficult to uniformly disperse within the thin layer. More generally, pigmented layers can suffer from greater haze and reduced clarity even in applications (for example architectural window films) with a relatively moderate, low and even minimal levels of desired darkening.
A continuing need therefore exists in the art for an optical product that meets all the haze, clarity, UV-stability and product longevity demands of current commercial window films, while also being manufacturable by an environmentally friendly, aqueous-based coloring process performed preferably at ambient temperatures and pressures.